1. Field of Invention
The invention relates to an electroacoustic transducer and more particularly to a transducer with a membrane mounted flush with the sound reflecting surface (boundary-layer microphone).
2. Description of The Related Technology
The periodical "Funkschau", No. 16, 1985, pages 43 and 45 describes a transducer with a flush mounted membrane. Standing waves are always formed in a room by the superposition of a directly incident sound field on sound waves reflected from walls. This leads to frequency and location dependent acoustic pressure maxima and minima. The sound waves have a pressure maxima directly in front of a sound reflecting surface. The velocity component perpendicular to the surface disappears as the incident and reflected waves are superposed in the same phase. The acoustic pressure in front of the surface is therefore twice a high as in the free sound field. This effect is utilized in boundary-layer microphones (Funkschau, No. 16, 1985, pages 43-45), in which a miniature electric transducer is mounted on a flat, thin sound reflecting mounting plate. Elastic feet may be provided on the bottom side of the mounting plate to immobilize the plate on a floor, wall or other sound reflecting surfaces. The sensitivity of the transducer is raised by 6 dB relative to the free field due to the increase in sound pressure immediately on the surface up to a double value. This doubling of the acoustic pressure occurs for frequencies where the surface is large compared to the length of the sound waves.
Either circular, square or rectangular reverberant plates are used for known boundary-layer microphones. The transducer element is usually centrally mounted. The plate edges are usually chamfered or irregularly rounded.